Tag Archives: Snowstorm


The most important weather model runs, based on global weather balloon (“radiosonde”) upper air measurements, are done twice a day— at 7 PM EST (00 UTC, also called 00z)  and 7 AM EST (12 UTC, also called 12z).

These new upper air measurements often account for any significant change in the model forecasts. The latest NAM-NEST has a decrease in snow totals due to significantly more sleet.

Here’s the latest 00z model data, based on these new upper air measurements.

The 00z HRRR (High Resolution Rapid Refresh) shows snow developing between 12 noon and 2 PM Wednesday and ending 7-10 AM Thursday. It will become very WINDY Wednesday night.

The HRRR continues the trend for sleet to mix in with the snow in a large portion of the area during Wednesday evening, and then change back to snow before ending.

HRRR sleet, rain snow precip type.  9 PM Wednesday  (Click on image for a larger view.)


Despite the changeover to sleet at times, the latest HRRR has increased snow totals by Thursday morning—

HRRR snow total forecast  (Click on image for a larger view.)


The 00z NBM (Model Blend) shows a similar snow total forecast—

NBM Snow Totals  (Click on image for a larger view.)


Several models are showing wind gusts in excess of 50 mph Wednesday evening.

Speaking of model forecast changes, the NAM NEST just available has more sleet and much less snow

NAM NEST Snow  (Click on image for a larger view.)

I think the NAM-NEST may be too warm with too much sleet.   We’ll have to see the trends into tomorrow morning.

Right now I’m sticking with the HRRR and NBM snow forecasts above.


Here, just northwest of the city limits, the precipitation has changed to rain with a pause in the action.  A lot has to happen to get to snow totals that the models have forecast!  Perhaps things have been a bit over-forecast?

This morning’s GFS model continues with the same scenario all the models have been forecasting; heavy precipitation with dynamic cooling to allow temperatures to drop and accumulations to occur.

HOWEVER, the GFS Kuchera snow algorithm makes this storm a bust—

Snow totals based on Kuchera
Latest GFS Snow totals based on the Kuchera algorithm!

As mentioned last night, I’m not big on the Kuchera snow algorithm; it tends to seriously underestimate snowfall in past storms.  (Last Friday’s storm had the Kuchera algorithm forecasting NO snow!)

Radar trends show a significant increase in precipitation rates in NJ, rotating westward into Pennsylvania.    Despite all forecast parameters as snow, things have to change soon for this forecast to not be a bust.  And as I type this, precipitation has changed back to snow!  So let’s see what happens.

BTW, here’s the latest HRRR (High Resolution Rapid Refesh) experimental forecast snow totals—

HRRR experimental
Experimental HRRR forecast snow accumulation from 12Z today


Philly Snowstorm Update 9AM

Here’s the very latest weather model data about the Philly snowstorm.  The morning’s NAM data just became available. It shows a QPF of 1.35 inches water falling as snow between 7AM and 7PM.

Here’s the latest NAM 10:1 snow totals map.  (Important- this map shows additional snow totals that need to be added to any snow that has already accumulated by 7 AM this morning.)

Philly Snowstorm totals remaining
NAM (12Z model run) snow accumulation based on 10:1 ratio)

Temperatures are expected to fall to 32-33 as precipitation rates increase during the late morning and early afternoon.    Expect heavy snow at times.  Temperatures may rise at the end of the storm to 34!

In my neck of the woods, we’re already seeing the difficulties of predicting snow totals for a March snowstorm.  Some surfaces and sidewalks have little accumulation, while our wood deck has about 2 inches right now.

[su_note note_color=”#ebf2d9″]With these issues with March snowstorms, we’re always predicting potential accumulations. Melting due to warm ground surfaces, radiant heat from solar insolation through clouds and snow compaction due to wetness can greatly affect final totals. Much depends on dynamic cooling to bring surface temperatures down.

Put another way, the precipitation rate is going to need to pick up considerably soon to meet the QPF values and the snow totals predicted by the NAM. [/su_note]



The 1 AM model runs continue with the scenario that the heaviest precipitation is just getting started.  The NAM has an additional QPF  1.20 inches water from now through 7 PM this evening.  The GFS, has a somewhat less but similar QPF through 7 PM.

So the snow is just getting started; the storm is still near Virginia/Maryland right now —

Current RAP model showing position of low pressure
Current RAP model showing current position of low pressure at 8AM Wednesday

So the forecast remains on-track.  With heavier precipitation rates, temperatures should continue to drop to about 32 degrees.

[su_note note_color=”#ebf2d9″]Current forecast is 9-16 inches.  The NAM 10-1 snow map below is good estimate. [/su_note]

NAM Snow Predictions
NAM Snow Totals Prediction from the 06Z model run

Hey, I kept hearing on the radio yesterday that the we were going to get snow but the high temperature on Wednesday was going to be about 40!  Why do they say such things??

Who knows!

I’ll update with the latest model data after 9 AM today.



The tonight’s NAM model data is becoming available.  Here are the trends:

Higher QPF values- precipitation falling as water has increased significantly to 1.80 inches water, in some areas, over 2.0 inches water!

The predicted vertical thermal profile is much COLDER; indeed, critical “thickness levels” and temperatures now support snow into New Jersey where previously it was thought to be mostly rain or rain mix.

With these changes, there’s been a significant increase in predicted snow totals!  Here are the NAM 10:1 snow totals by area:

NAM Final predicted snow totals 10:1 ratio

There are other algorithms that try to take into account other factors such as melting.  One is called the Kuchera Snow Algorithm.

Kuchera algorithm
NAM Kuchera Algorithm Snow Accumulation

I don’t generally find the predefined snow algorithms that useful or accurate.  I believe the 10:1 simple algorithm will do best here and may understate snowfall at times. An average of the two might also prove correct for this storm.

The reason I’m presenting both predefined snow algorithms is to show that significant snowfall is now expected in Philadelphia, NJ as well as in PA.

[su_note note_color=”#ebf2d9″]So we’re looking at 9-17 inches of snow, based on your location, as shown on the above maps.[/su_note]

Timing:  Precipitation as rain is starting as scheduled (9PM) and will mix with and turn to snow by 1-2 AM in western sections and a bit later along the I-95 corridor.   Heaviest snow during the morning and afternoon hours on Wednesday, ending about 7PM or so.

Temperatures are expected to fall to about 32-33 degrees by 2-3 AM with dynamic cooling that will occur with heavier precipitation on Wednesday

Winds will be 15 to 20 mph with gusts near 25 during the day on Wednesday.

The GFS model data becomes available about 10:45 PM.  I expect the GFS QPF values to be less than the NAM values; it always is.  I still think we’re talking about a major snowstorm for Philadelphia and the immediate areas.  This is a high confidence forecast.

I’ll amend this post with the GFS data if it presents a problem with this forecast.

[su_note note_color=”#ebf2d9″]10:45 PM – Tonight’s GFS model data is similar to the NAM. QPF values 1.43  inches water with distribution of precipitation maximum centered around I95, similar to the NAM.  I believe the NAM 10:1 snowfall map above  is a good estimate. [/su_note]